The art is good; I really like how the flames on the city look. I appreciate the very generously large area around planes where you can hit them, since the game would probably be nearly impossible without it.
It might feel a bit more natural if the cannon were aimed with the mouse, though controlling it with the keyboard does give it a "heavier" feel, so maybe that's intentional.
I would have liked if there were a bit more visual or audio feedback for firing, aiming, and changing the fuse time. In particular, I found it difficult to figure out the fuse time at a glance. Maybe if the number changed color as it got higher?